As known by those skilled in the art, ideally two fibers (i.e., optical fibers) being spliced together should have the same numerical aperture (“NA”). This is true even if the fiber cores are different size. Thus, NA is a critical parameter when splicing fibers.
Generally, in fiber optics NA is a dimensionless number that characterizes the range of angles over which the fiber can accept or emit light, and is given by the formula:NA=√{square root over (n11−n22)},where n1 is the refractive index of the fiber core and n2 is the fiber index of the cladding. The fiber core and cladding tend to have slightly different make ups, which results in the different refractive indexes. See FIG. 1 for an example.
Selectively changing the NA of an optical fiber typically involves changing the refractive index and diameter of the core. This can be accomplished by applying a flame having a width of about 5 mm-6 mm to a fixed fiber. The fiber is heated at its melting temperature for a relatively long time, i.e., several minutes or more, until the cladding and core start to blend—thereby changing the refractive index of the core by diffusion of dopants therein. An end of the fiber can also be pulled, so that the diameter of the fiber is also selectively reduced, also referred to as “tapering.”
The heated portion is cleaved at the center of the thermal profile, which is generally Gaussian. Therefore, assuming the flame has a width of about 5 mm, a cleave can be made at about 2.5 mm from each end of the thermal profile. See FIG. 2.
This process works relatively well for an individual fiber. However, because using such a flame takes several minutes or more to achieve the necessary diffusion, this cannot be done quickly. Slow diffusion means less yield and higher cost.
Also, the stationary flame with a fixed flame width presents limitations with respect to achievable thermal profiles that can be used for diffusion. And the same flame is used regardless of the core diameter.